Effect of protein binding on charge transfer in DNA: A simple model based on the superexchange mechanism

Abstract

The effect of protein binding on the electronic coupling between distant redox centers in DNA is investigated in DNA–protein complex systems using the superexchange formalism. The systems (bridges) studied are described by a tight-binding electronic Hamiltonian in which site orbitals interact with one another through an exponentially decaying function of distance. Based on the "continuous-medium approximation," previously developed for large homogeneous three-dimensional systems (J.-M. Lopez-Castillo et al. J. Phys. Chem. 99, 6864 (1995)), the intervening bridge is defined by a unique dimensionless parameter Γ /E that controls the distance dependence of the electronic coupling. Here, E is the energy separation between the orbitals of the bridging medium and the redox sites (tunneling energy), and Γ is the electronic bandwidth of the bridge taken as a continuous medium. It was found that, for a given value of (Γ/E)DNAfar from the DNA's resonance conditions and for (Γ/E)proteinvalues near the protein's resonance conditions, the electronic coupling is independent of the donor–acceptor distance when the acceptor lies within the "recognition region" of DNA. Moreover, when the redox centers are located on both sides of this region, the electronic coupling is many orders of magnitude larger than it should be, far from the protein's resonance conditions.Key words: DNA, DNA–protein complexes, long-range electron and hole transfers, electronic coupling, superexchange mechanism, energetic control, continuous-medium approximation.